Flame detector system using a lightly loaded glow discharge detector tube



3,286,093 LIGHTLY LOADED GLOW DISCHARGE DETECTOR TUBE Original FiledNov. 2, 1962 Nov. 15, 1966 L. F. GILBERT FLAME DETECTOR SYSTEM USING A 5Sheets-Sheet l 7 INVENTOR.

LYMAN F. GILBERT ATTORNEY 3,286,093 IGHTLY TUBE 5 Sheets-Sheet 2 L. F. GERT FLAME DETECTOR SYS USING A L LOADED GLOW DISCHARGE DETECTOR OriginalFiled Nov. 2, 1962 Nov. 15, 1966 MQE ................. a MOI J 4 4 4 JNQE INVENTOR. LYMAN F. GILBERT [WA A r ATTORNEY Nov. 15, 1966 F. GILBERT3,286,093

FLAME DETECTOR SYSTEM USING A LIGHTLY LOADED GLOW DISCHARGE DETECTORTUBE Onginal Filed Nov. 2, 1962 5 Sheets-Sheet f5 INVENTOR. 6 LYMAN F.GILBERT ATTORNEY United States Patent 3,286,093 FLAME DETECTOR SYSTEMUSING A LIGHTLY LOADED GLOW DISCHARGE DETECTOR TUBE Lyman F. Gilbert,Hazardville, Conn., assignor to Combustion Engineering, Inc., Windsor,Conn., a corporation of Delaware Continuation of application Ser. No.235,098, Nov. 2, 1962. This application July 22, 1966, Ser. No. 567,3107 Claims. (Cl. 25083.6)

This application is a continuation of my copending application SerialNumber 235,098, filed November 2, 1962, now abandoned.

This invention relates generally to electrical flame detectors operativeto continuously detect the presence or absence of a flame within afurnace. The invention is particularly concerned wth detectors of thetype employing glow discharge sensing tubes which are positioned to seethe flame and receive their firing energy from the flame.

In accordance with the present invention there is provided an electronicflame detector which has a minimum of circuit components and which isextremely reliable in its operation. The detector is far more sensitivethan prior art detectors of the same general type and is so designedthat only the detector tube, i.e., the glow discharge tube, need bemounted on the furnace.

The flame detector of the invention includes what may be termed atransmitter that generates a pulse signal incident to avalancheoccurring in the detector tube which pulse is separated from the AC.supply and coupled into what may be termed a receiver as set inputsignal therefor and with the output or load of the receiver beingenergized as long as the pulse output of the transmitter is produced.

It is an object of the invention to provide an improved electronic flamedetector system.

A further object of this invention is to provide such an improved systemutilizing a minimum of electrical components and having a maximum ofreliability.

Another object of the invention is to provide such a flame detectorsystem which is extremely sensitive to the detection of a flame in afurnace and which is so arranged that a minimum of components arerequired to be mounted on the furnace. 7

Other and further objects of the invention will become apparent to:those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises anarrangement, construction and combination of the elements of theinventive organization in such a manner as to attain :the resultsdesired as hereinafter more particularly set forth in the followingdetailed description of an illustrative embodiment, said embodimentbeing shown by the accompanying drawingswhere- FIGURE 1 is a circuitdiagram of the present invention;

FIGURE 2 depicts the wave form produced in the transmitting circuitincident to the firing of the glow discharge tube where the tube is notheavily loaded;

FIGURE 2A depicts the wave form produced in the transmitting circuitincident to the firing of the glow dis-charge tube where the tube isheavily loaded;

FIGURE 3 illustrates the wave form that is the output of the transmitterwhich is coupled into the reeciver as the input therefor;

FIGURE 3A illustrates the wave form produced in the receiver through theswitching action effected by the pulse output of the transmitter. Alsoillustrated in dotted lines in FIGURE 3A is the integrated wave formthat appears across the load or output of the receiver;

FIGURES 4, 5, 6 and 7 illustrate varying circuit designs for thereceiver and which may be utilized with the present invention.

Referring to the diagram of FIGURE 1 there is provided a source of AC.potential identified as 10 and whichis effective to power thetransmitter 12 of the flame detector through the transformer 14, withthe transformer stepping up the voltage of the source, such as from 118V. AC. to 750 V. AC. The frequency of this supply may be the readilyavailable 60 cycles per second. Connected across the secondary 16 of thetransformer are one or more glow discharge tubes UV. In series with thistube UV are fuse 18, resistors R and R and capacitor C (R may also be asmall transformer or inductor.) With this circuit the tube UV is lightlyloaded so that it may partially conduct and recover, returning to itshigh impedance state. The operation thus obtained may be that shown inthe FIGURE 2 wave form illustrations wherein the voltage spikes areshown riding on the 60 cycle A.C. supply of the UV tube and wth thesespikes representing the surges or pulses produced incident to firing(avalanche) of the tube. These pulses produce shock excitation of thecircuit with the ringing or alterna tions at the trailing edge of thepulse gradually being damped out.

The wave form depicted in FIGURE 2 illustrates the UV tube operating outof saturation, i.e., so that firing of the tube is not uniformlyeffected during every alterna-- tion of the supply potential but with arandom firing being effected. It will be understood that the tube mayalso be operated in saturation where the tube fires with everyalternation of the supply. The criterion as to whether or not the tubeoperates in or out of saturation depends upon its position withrelationto the flame being sensed plus the voltage applied to the UV tube. Theenergy being sensed follows the inverse square law of radiation andtherefore the signal strength will vary inversely as the square of thedistance between the sensing tube and the flame. The scanning anglewhich the sensing tube has to view the flame also affects the energyreceived directly as the square of the diameter of the circle viewed atthe point of the flame envelope.

The sensitivity of the tube may also be adjusted varying the supplyvoltage to the transmittter circuit by adjusting the supply voltage oftransformer 14. Thus,

- depending upon the relative position of the tube to'the mechanismacross a threshold potential with what is termed a Townsend avalancheoccurring at breakdown. Thus when a photon of proper energy content,i.e., proper wavelength, enters the tube it causes ionization of thegases therein resulting in a very rapid acceleration of electrons acrossthe potential gap of the detector tube. In analyzing the operation ofthe circuit in which the glow discharge tube is disposed it was foundthat the speed of the pulse produced by the Townsend avalanche and whichis reflected back through the circuit (through shock excitation) has aduration of less than .5 micro-' second (corresponding to a frequency ofbetter than 2' megacycles). It was thus determined that this pulse couldbe readily separated from the supply voltage and employed in thereceiver of the flame detector as the input signal therefor therebyrendering the transmission circuit fail-safe with regard to any shortingor opening ofthe circuit or the components thereof.

In the transmitting circuit the RC network established by R C increasesthe time during which the very short pulse signal is effective. Whilethis is not essential it does add tothe reliability of operations of thecircuit. This pulse signal is separated from the 60-cycle Supply bymeans of the separating or filter network identified as R and C R ispreferably a wire wound resistor across which a voltage drop isdeveloped by the pulse current established at avalanche (R can also be atransformer or inductor). C is of such a value as to pass the highfrequency pulse while rejecting the 60-cycle source. The separated pulsevoltage is thus established between the conductor 20 and ground. FIGURE3 depicts this separated wave form with the negative going pulse beingattenuated by shunting to ground through a suitable diode connected toground for purposes concerned with operation of the receiver as laterdescribed. The fuse 18, which may be & amp., will cause an opening ofthe transmitter circuit incident to an overload as a result of thedeveloping of a short.

In the event that the UV tube is heavily loaded rather than lightlyloaded as would be the case if the capacitor C were eliminated from thecircuit or made very large, then the tube would not be able to recoverafter the avalanche and would continue to carry current until the supplyvoltage dropped below the cut-off level of the tube. FIGURE 2Arepresents the wave form produced in the transmitter under' suchconditions. The separated signal leaving capacitor C would still be ofthe nature shown in FIGURE 3.

The lightly loaded circuit is several times more sensitive than theheavily loaded circuit. In the heavily loaded circuit high energyphotons are required to produce sufiicient ionization of the gaseswithin the tube to cause an avalanche. In the lightly loaded circuitphotons of much lower energy content but of the proper wave length. aresufficient to partially ionize the tube thus causing the pulse to beemitted (avalanche) but the tube is capable of recovering and thus doesnot lose control. The wave form of FIGURE 2 illustrates the frequency ofoperation with a lightly loaded circuit. The wave form of FIGURE 2Aillustrates the lower frequency of avalanche occurring from the sameenergy source. It is for this reason that the lightly loaded circuit ispreferred thereby taking advantage of the greater senitivity obtainedtherewith. However, it is to be understood that the heavily loadedcircuit although less sensitive is within the purview of the inventionwith this circuit distinguishing and separating the signal producedincident to sensing the flame from supply potential of the transmitter.

With this transmitter circuit, should there be a short of the leads tothe UV tube, to which prior art organizations have beensusceptible, orshould there be a short or an opening of any of the components of thecircuit the normal pulse signal produced by the transmitter andindicating the presence of a flame will no longer be present whereforethe transmitter is fail-safe or in other words produces a no flamesignal incident to a rnalfunction of the circuit. If the UV tube shorts.the pulsesignal will no longer be produced and moreover fuse 18 willblow. If R and/or C short, no detrimental effect will be producedalthough the duration of the pulse will be decreased and the sensitivityof the circuit will be decreased. If R or C open, the signal will nolonger be produced. If R shorts, the signal will no longer be producedthrough C and if C shorts, no detrimental effect will be produced unlessthe corresponding series connected capacitor in the receiver shorts inwhich case fuse 18 will blow and the signal no longer produced, Shouldthe transformer short, the signal would no longer be produced.

7 The transmitter of the flame detector of the invention thus produces apulse signal of very short time duration which pulse signal is separatedfrom the supply which powers the transmitter and is coupled into thereceiver as the input therefor. The transmitter is an extremely simpleelectrical circuit including the fuse 18, tube UV, R C forming a pulseshaping or delay network to increase the duration of the pulse and R allof which are in series and connected across the secondary 16 of thetransformer 14. R in cooperation with C forms the signal separatingnetwork for separating the pulses from the transformer supply potential.Several of the UV tubes may be connected in parallel in a singletransmitter with two such tubes being illustrated. This would bedesirable when there may be two or more burners or where it is desiredto view the flame of a single burner from more than one location.

More than a single transmitter may be operated from the secondary 16 ofthe transformer. In the illustration of FIGURE 1 two transmitters areconnected in parallel across the transformer secondary with the secondtransmitter being identified as 12' and containing the same componentsas previously described with transmitter 12. This number can beincreased so that eight or more flame detectors may be operated from asingle transformer if desired. This is of considerable advantage inlarge furnace installations wherein as many as 24 flame detectors areutilized with a single furnace. When multiple transmitters are employedwith a single transformer, the capacitor C is connected across thesecondary as a filter to prevent the signal spikes of one transmitterfrom interacting on other signal separators and UV sensing tubes.

The receiver of the flame detector of this invention receives theseparated pulse signal from the transmitter as its input signal with thepulse signal actuating a switching device in the receiver and with thereceiver being operative to develop a driving or power signal that isapplied to a load for activating the same. In the illustrativeorganization of FIGURE 1 the pulse signal output of the transmitter iscoupled into the receiver through the capacitor 0.; which acts tofurther filter any 60-cycle supply of thetransinitter from the signalwith the capacitors C C providing a safety feature in that satisfactorybe obtained if one of these two capacitors should become shorted. Thesupply potential for the receiver in this illustrative embodiment is ahalf lwave supply and is obtained from the 60-cycle 118 volt source 10.Half wave rectification of the supply is obtained by the diode D Thereceiver, in effect, switched on and off by the Silicon ControlRectifier identified as SCR. The negative voltage pulse received fromthe transmitter are shorted to ground through diode D while the positivepulses are operative to gate the SCR on with the lead 20 being connectedto the gate of the SCR. Once it is gated on the SCR completely losescontrol and the SCR conducts until its anodeto cathode voltage isreduced to a very low value. Accordingly, the SCR is gated on inaccordance with the pulse output of the transmitter (this being a randomrepetition rate when the tube UV is operated out of saturation)intervals when no current is flowing through the receiverwith "R and Cforming a tank circuit. Thus the relay is continuously engaged as longas the pulse signal output of the transmitter is received by thereceiver. The R C network acts as an integrator with regard to thedriving signal produced in the receiver providing partial integration ofthis signal.

FIGURE 3A illustrates in solid lines the driving pulse produced throughthe triggering of the SCR while the dotted line wave form illus- 5trates the partial integration of this driving signal that is producedby the R network.

With the flame detector of this invention it is only necessary that theUV tube be mounted on the turnace and accordingly subjected to thesevere operating conditions with relation to temperature, vibration,etc. existing at this location. The UV tube may be connected to theremaining components of the receiver through a twowire coaxial cablewhich may be more than 500 feet in length. The coupling of the outputsignal of the transmitter to the input of the receiver may be through atwo-wire coaxial cable that may be more than 200 feet in length therebypermitting the receiver to be located where desired.

In lieu of applying the positive going output pulses of the transmitterdirectly to the gate of the SCR a switching transistor may be employedto receive the output pulsesof the transmitter. FIGURE 4 illustrates areceiver circuit so arranged employing transistor Q which is switched onby the negative going pulses of the output of the transmitter (in thiscircuit, arrangement the shunting diode D is not employed). Thisreceiver of FIG- URE 4 has a full wave power supply employing a centertap transformer 23 with diodes D and D and with the trnasistor Qconnected, as disclosed, across this power supply. The input signal. toQ is applied to the base to which lead 20 is connected and during thetime that the transistor is switched on capacitor C is charged to adesired value. The resistor R connected between the collector of Q andthe gate of SCR operates to limit the .gate current to the SCR. Diode Dat the emitter connection is effective to provide reverse bias to QCapacitor C increases the time constant of the gate pulse, which pulseapplied to the SCR in contrast to the FIG- URE l arrangement, is ofconstant value and will be amplified over that applied to the input atthe base of the transistor Q The driving signal produced by the firingof the SCR is similar to that produced in the FIGURE 1 organization withthe actuation oractivation of the load R and the function of thecapacitor C being the same as that in the FIGURE 1 arrangement.Capacitor C may be connected across the SCRin the manner disclosed toaid the SCR in recovering control of the anode 22 to cathode 24 voltagethus making the circuit more reliable in operation. Such a capacitor maybe employed if desired in the other receiver circuits of the inventionemploying an SCR. v The receiver circuit of FIGURE 4 has a low voltagesupply for use with the transistor Q thus transformer 23 may step thevoltage down from 118 V. AC. to 24 V. AC. This circuit may be convertedto high voltage operation by adding'a regulating diode and a dropingresistor and by changing the ratingsof the other components of thecircuitfor high voltage application. FIG URE 5 shows the receivercircuit thus converted. The regulating diode D in combination with thedropping resistor R is operative to limit the voltage applied across theemitter and collector of the transistor with a -volt Zener diode havingproved satisfactory 'for this purpose. The operation of this receiver issimilar to that of FIG- URE 4 with the transistor Q being used in pulseduty only and providing a switching action in accordance with thereceipt of the varying repetition rate pulses from the transmitter tothus produce pulses to trigger the SCR. The supply voltage to thereceiver circuit of FIGURE 5 may be 120 volts with full waverectification and with a center tap secondary of transformer 23' beingillustratively shown to obtain this supply. The diodes D and D areemployed with the center tap circuit arrangement to provide thenecessary rectification for the full .wave supply.

In lieu of employing a full wave power supply as disclosed in FIGURE 5 ahalf wave supply may be employed for this receiver circuit, with FIGURE6 showing such an arrangement wherein the supply voltage as ob- 6 tainedthrough transformer 25 may 'be volts. In this FIGURE 6 organization avoltage pulse is developed across the resistor R when the transistor Qis switched on with the pulse being etfective to trigger the SCR of thereceiver and accordingly providing a driving signal as described withrelation to the circuit of FIGURE 1. Note that the higher voltage SCRs(FIGURES 5 and 6) require shorter gating times and therefore C (FIGS. 4and 5) may be omitted as is done in the FIGURE 6 circuit.

In the circuits of FIGURES 4, 5 and 6 the transistors Q may be replacedby a uni-junction if desired with these components being interchanged inthese circuits.

- In each of these circuits of FIGURES 1, 4, 5 and 6 the driving sign-a1produced in the receiver is obtained through the triggering of the SCR.While this provides a very simple and reliable receiver, the receivercircuit is not limited to the use of an SCR and other circuit designswhich will produce a driving signal in the receiver for energization ofthe receiver load may be utilized. FIGURE 7 illustrates a transistorizedreceiver employing a switching transistor and two transistorizedamplifying stages. In this circuit the load relay is a low voltagedevice operating within voltages comparable to the transistor state ofthe art. The input signal is coupled into the receiver at the base ofthe transistor Q turning the transistor on and off in accordance withthe pulses as previously described. The power supply for thetransistorized receiver may be half wave or full wave voltage supplywith a half wave supply being illustrated. For example, the powertransformer 27 may have 118 volts 60-cycle primary source producing 12.6volts AC. on the secondary. Diode D (FIGURE 7) rovides the necessaryhalf wave rectification. The circuit of transistor Q is an emitterfollower so that the output closely follows the inputbut with a currentgain approximately that of the transistor gain. The emitter-collectorcurrent is limited by the resistor R C acts to store some of the energyof Q to maintain Q turned on between signal pulses to Q The inputcircuit of this transistorized receiver is thus substantially the sameas the inputs of the receiver circuits of FIGURES 4, 5 and 6 withtransistor Q being used in pulse duty and with an amplified partiallyintegrated pulse (through the action of C and R being applied to thedirect coupled amplifier stages of the receiver. In the circuit ofFIGURE 7 a reverse bias resistor R7 is provided to keep Q turned off athigh temperatures with the reverse bias voltage being provided throughdiode D and capacitor C Transistors Q and Q; are connected in commonemitter amplifier configuration. The input is applied to the base -of Qwith the gain of Q being limited by R connected between the collectorand the source of potential with the emitter of Q being connected withthe base Q The emitter of Q; is connected with the source while R isconnected with the collector. As'in the case of the transistor Q reversebias loading is provided for Q and Q; to compensate for high ambienttemperatures with the bias for Q being provided through R and the biasfor Q being provided through R and with the source of potentials forthis bias being obtained through D and Q, as in the case of the reversebias Q The output of the amplifier is a varying signal which is appliedto the load R across which is connected the holding capacitor C with theoperation of the load relay being the same as that obtained with theFIGURE *1 receiver circuit.

The signal as impressed across the load of the receiver in each of themodifications is integrated sufiiciently so that it is maintained abovea sufficient level so long as the input of the receiver is transmittedthereto from the transmitter to activate the load. For instance, in ahigh voltage receiver circuit such as that of FIGURE 1 and 6 employing ahigh voltage relay the voltage across the relay may vary, for examplefrom 80 to volts with this variation being at a random rate but sincethe relay requires a considerably less potential than 80 volts such as45 volts, to pull in and 25 volts to-drop out, the relay will bemaintained engaged as long as the output signal of .the transmitter iscoupled into the receiver.

In the event that there is a flame out and the transmitter signal thusterminated of the load relay of the receiver will drop out in a fractionof a second. Tests have shown that the relay drop out time may bebetween .1 and 1.5 seconds after a sudden and complete loss of flame.The amount of time after flame out to obtain a flame out indication atthe receiver load, or in other words to obtain drop out of the relay,will depend upon the circuit components and primarily the capacitorswith the design being such as to maintain drop out in a very shortperiod of time.

In the operation of flame detectors it is desired that the sensingelement respond only to energy emitted by ,the flame and not otherenergy sources such as the energy admitted by hot refractory or hottubes or glowing carbon that may be in the vicinity of the flame.Accordingly, the detector of the present invention employing a glowdischarge sensing tube utilizes a tube which responds to energy thatlies generally within the spectrum range of 2000 to 3300 angstroms.Within this range the sensing tube will respond to photons admitted bytheflame and will not respond to energy emitted from other sources suchas glowing refractory. The wave lengths of the photons emitted by theseother sources are somewhat above 3300 angstroms. Accordingly, thesensing tube operates in the ultraviolet spectrum range. In thisdesignated range, i.e., between 2000 and 3300 angstroms, there exists aserious problem with regard to the detection of flames pro- .duced byburning of coal. The energy emitted from the coal fire in this spectrumrange is rather limited being much less than that obtained with gas oroil fires and, accordingly, in order to detect coal fires by means of aglow discharge tube operating within this general spectrum, it isnecessary that a very sensitive system be employed with so-calledultraviolet flame detectors of prior art design and as employed priortoapplicants invention being incapable of satisfactory operation withcoal fires.

As explained hereinbefore when the UV tube is lightly loaded it is verysensitive, producing avalanches from relatively low energy photons andwith the tube recover ing after each such avalanche. By utilizing theflame detector circuit of this invention with the UV tube not beingheavily loaded, it has been possible to detect the presence or absenceof flame in a coal fired installation in an entirely satisfactory mannerwith the result being accurate and dependable. Accordingly, with theinvention there is provided a flame detector system which operatessatisfactorily on either gas, oil or coal firing and which isselectively responsive with regard to the energy spectrum range overwhich it is sensitive so that false indications of the presence of aflame are avoided.

The load as identified as R in the various receiver circuits may take avariety of forms. It may be a relay which can operate a switchingmechanism such as closing a .switch when energized to provide an alarmeither audible or visible or open a switch when energized and close aswitch when de-energized to activate a suitable alarm. A relay may openone switch when energized and close another upon de-energization closingsaid one switch and opening said other switch with indicators beingenergized upon closing of the respective switches. Furthermore, the Rmay not be a relay at all but rather may be the input load to a computeror signal static de- .vice, not requiring an intermediate relay. Stillfurther, the R may be a plurality of loads for instance relays,computers, lights, etc., all driven simultaneously from the samereceiver.

While the potential source for the transmitter and receivers, asdescribed hereinbefore, is stated to preferably be a 60-cycle per secondsource, this is by way of example only and is utilized merely because ofits ready availability. It will be understood that the source for thetransmitter and receiver may be any desired frequency so long as properoperation of the components of the circuits, for example the tube of thetransmitter and the SCR of the receiver, may be obtained and so long asthe period of the source is substantially longer (many times) than thetime duration of the pulse produced in the circuit incident to avalanche(well over a millisecond) occurring in the tube so that the signalseparator network can separate the pulse signal train from the AC.source. From known pulse techniques it is considered that a differenceof ten to one is desirable and will provide for ease of separation ofthe signals. In other words, the effective frequency of the pulse-shouldbe ten times the frequency of the supply. It should be noted that withthe tube lightly loaded so that it immediately returns to its highimpedance state after avalanche, the source for the transmitter may be aD.C. potential although for ease of circuit design an AC. source ispreferred.

Accordingly, with the present invention there is provided a flamedetector utilizing a glow discharge tube as the sensing element andhaving a circuit which is simple and reliable with the detector beingconsiderably more sensitive than so-called UV flame detectors ofheretofore known design and with it being necessary to mount only thedetector tube on the furnace within which the flame is to be detected.

While I have illustrated and described a preferred embodiment of myinvention, it is to be understood that such is merely illustrative andnot restrictive and that variations and modifications may be madetherein without'departing from the spirit and scope of the invention. Itherefore do not wish to be limited to the precise details set forth butdesire to avail myself of such changes as fall within the purview of myinvention.

What I claim is:

1. A flame detector comprising in combination a source of potential, aglow discharge sensing tube connected to the source of potential andhaving in series therewith an integrating network and a load impedanceacross the latter of which an output spike signal is developed incidentto avalanche occurring in said tube, a receiver having an input and anoutput and including a source of potential, a silicon controlledrectifier having its anode and cathode across said source, a loadcontrolled by said silicon controlled rectifier, means providing a highimpedance to said source of potential connected to the sensing tube anda low impedance to the spike signal capacitively coupling said outputspike signal into the input of the receiver with the silicon controlledrectifier being gated on in response to such coupling of said signal.

2. The detector of claim 1 wherein said receiver includes a transistoroperated in pulse duty only with the output spike signal being coupledto the base of said transistor, means for increasing the time durationof the signal produced by application of said spike signal to saidtransistor and operative to couple this thus modified signal to the gateof the silicon controlled rectifier.

3. The detector of claim 1 wherein said output spike signal is coupleddirectly to the gate of the silicon controlled rectifier.

4. The flame detector of claim 2 including another transmitter connectedto the source of potential and in parallel with said first transmitter,and a capacitor shunting said source of potential, said transmitterbeing adapted to have similar receiving means into which its outputsignal is coupled.

5. A flame detector comprising in combination a source of potential, aglow discharge sensing tube connected to the source of potential andhaving in series therewith a load impedance across which an output spikesignal is developed incident to avalanche occurring in said tube, areceiver having an input and an output and including a source ofpotential, a silicon controlled rectifier having its anode and cathodeacross said source, a load comprising a relay in series with the siliconcontrolled rectifier and across which a holding capacitor is connected,means providing a high impedance to said source of potential connectedto the sensing tube and a low impedance to the spike signal capacitivelycoupling said output spike signal into the input of the receiver withthe silicon controlled rectifier being gated on in response to suchcoupling of said signal.

6. A flame detector comprising a glow discharge sensing tube operatingin the ultra-violet spectrum range, an electric circuit into which saidtube is connected and including a source of potential applied across thetube, said circuit having an impedance resulting in a lightly loadedtube so that it recovers immediately after avalanche whereby a pulsesignal corresponding to a high 10 frequency is produced with thefrequency thereof being substantially higher than that of the supply,means for separating the pulse signal from the supply, means responsiveto this separated signal and operative to produce a driving signal.

7. The flame detector of claim 6 wherein the means for producing saiddriving signal includes a silicon controlled rectifier connected to begated on by the separated pulse signal.

No references cited.

RALPH G. NILSON, Primary Examiner.

A. R. BORCHELT, Assistant Examiner.

1. A FLAME DETECTOR COMPRISING IN COMBINATION A SOURCE OF POTENTIAL, AGLOW DISCHARGE SENSING TUBE CONNECTED TO THE SOUCE OF POTENTIAL ANDHAVING IN SERIES THEREWITH AN INTEGRATING NETWORK AND A LOAD IMPEDANCEACROSS THE LATTER OF WHICH AN OUTPUT SPIKE SIGNAL IS DEVELOPED INCIDENTTO AVALANCHE OCCURRING IN SAID TUBE, A RECEIVER HAVING AN INPUT AND ANOUTPUT AND INCLUDING A SOURCE OF POTENTIAL, A SILICON CONTROLLEDRECTIFIER HAVING ITS ANODE AND CATHODE ACROSS SAID SOURCE, A LOADCONTROLLED BY SAID SILICON CONTROLLED RECTIFIER, MEANS PROVIDING A HIGHIMPEDANCE TO SAID SOURCE OF POTENTIAL CONNECTED TO THE SENSING TUBE ANDA LOW IMPEDANCE TO THE SPIKE SIGNAL CAPACITIVELY COUPLING SAID OUTPUTSPIKE SIGNAL INTO THE INPUT OF THE RECEIVER WITH THE SILICON CONTROLLEDRECTIFIER BEING GATED ON IN RESPONSE TO SUCH COUPLING OF SAID SIGNAL.